Disk drives are data storage devices having a stack of one or more magnetic recording disks mounted for rotation about a common axis. Data is stored on the disks in concentric tracks, and is read from or written to the disk surfaces by transducing heads. There is typically one read/write head per disk surface, although only a single head/disk surface combination normally reads or writes user data at any given time. All tile heads are ganged together common motion on an actuator, which moves the heads radially in and nut across their respective disk surfaces. In most modern disk drives the heads glide over the rotating disk surface at such a low height (&lt;1 .mu.m) that any dust or other contamination on the disk surface is likely to result in a head crash, loss of data, and possible damage to the head. For this reason, the disks, actuator, and heads are placed in a sealed unit known as the head-disk assembly (HDA).
The movement of the actuator is controlled by a servo system, utilizing servo information recorded on one or more of the disks. By reading this servo information, the actual radial positions of the heads can be determined, and after comparison with the desired head radial positions, control signals can be sent to move the actuator accordingly. Two types of servo system are commonly used. In the first, a dedicated servo system, one whole disk surface (a servo disk) is used exclusively to store servo information. A servo head constantly reads this information to provide a continuous signal indicating the position of the servo head with respect to the servo disk, and by extension, the location of all the other data heads on that actuator. In the second type of servo system, sectors of servo information are interspersed with sectors of data on each disk surface (this type of system is known as a sector servo). As a head follows the data track around, it regularly reads a fresh sample of servo information from each servo sector with which to control its position. In both types of servo system there are servo guardbands at the inner and outer diameters to indicate the radial limits of the data storage regions on the disks. Often one of the servo guardband regions is used as a landing zone for the head when the disk drive is powered down.
The servo pattern therefore comprises whole or sectors of concentric tracks. Each track is identified by an index number, allowing the radial position to the nearest whole number of tracks to be easily determined. Finer radial positional information is provided by an error signal derived from the relative strengths of signals from two or more adjacent servo tracks. The angular or azimuthal position of the head is obtained from circumferentially aligned transitions in the servo tracks, with some particular mark on the tracks being used to define the zero angle. The radial and angular measurements, corresponding to r and .theta. in polar coordinates, completely specify the position of the head over the disk surface.
The initial writing or the servo patterns onto the disk is an important step in the manufacture of disk drives. Clearly it is essential for the servo tracks to be written as accurately as possible, whilst at the same time great care must be taken to prevent contamination of the disk surfaces. Conventionally this has been achieved by using specialized servo-writing apparatus, with its own transducer head. The servo-writer is first used to write a circular clock track on the disk, providing a series of transitions indicating angular position around the disk. In one common method servo tracks at other radii are then written using the product head (i.e. the one that is actually part of the disk drive). Angular information, in the form of timing information telling the product head when to write certain transitions, is provided by synchronization with the clock track, which is simultaneously read by the servo-writer. The servo-writer typically includes a long-arm laser interferometer to maintain the product head at a constant radius. In this manner, circular servo tracks all referenced in angle to the clock track can be obtained.
While conventional servo-writers are capable of producing highly accurate servo patterns, they have a major drawback in terms of cost. This is firstly because the machines themselves are bulky and expensive, and require a large optical (vibration-isolation) table. Secondly, because the disk surfaces are exposed during the servo-writing process, this process must be performed in a clean room, which again adds to cost. Constraints on the amount of affordable clean room space or number or servo-writers limit the total throughput of a disk drive production line. For this reason, there have been various attempts to find simpler, less expensive methods or servo writing.
EPA 327207 discloses a method in which only one disk is servo-written conventionally. The remaining disk surfaces are servo-written after the HDA has been scaled by effectively copying the pattern from the disk that already has the servo pattern on. Likewise, U.S. Pat. No. 4,531,167 discloses a servo-writer that copies servo patterns onto disks inside the HDA from a master disk cartridge attached to the disk spindle outside the HDA. The master disk cartridge is removed once the disk drive has been servo-written. Neither of these techniques completely avoids using a clean room, and the method of U.S. Pat. No. 4,531,167 is only applicable to certain designs of disk drive.
An alternative to using a magnetic clock track is disclosed in IBM TDB Vol. 1-85, pp. 4877-4878 "Servowriter Reference Clocks From Shaft Encoder," which proposes using an optical guide disk with angular information marked on it attached to the main spindle. A clock track is read from the disk using a Moire fringe effect to supply timing information to the product heads which write the actual servo pattern. A similar approach is used with the IBM disk drive WD-L40, in which clock information is read from a paper pattern wrapped around the disk shaft. In both cases the optical guide has far fewer transitions than are required for the servo pattern, so that the frequency of the signal from the guide must be multiplied up to produce the servo pattern. It has proved difficult with such techniques however to obtain the high accuracy necessary for the ever-increasing density of data stored on disk surfaces.